1. Field of the Invention:
The present invention relates to voltage regulators. More specifically, the present invention relates to digital switching voltage regulators.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art:
Analog and digital switching voltage regulators are known in the art. In analog switching regulators the current shunted by a switch is proportional to the difference between an existing load voltage and a desired load voltage. The current shunting switch in analog regulators is typically a transistor operating in the linear mode. In certain applications the often high power consumption of analog regulators is reduced by "tapping" a power supply at less than the available supply voltage. In this manner the voltage across the transistor switch is reduced thereby reducing power consumption. However, this reduction in the transistor operating point voltage induces a corresponding reduction in the amount of power (equal to the product of tapped voltage and current) which may be shunted by the transistor prior to saturation. Hence, the regulation range of the conventional analog regulator is limited in proportion to this reduction in power handling capability.
In contrast, conventional digital regulators are typically not limited in current or power handling capability. Conventional digital switching regulators effect regulation within adjacent voltage "windows" centered about voltage setpoints sequentially offset from the desired load voltage. These digital regulators include an array of modules each including a window comparator centered about one of the setpoints. Each module typically includes a power supply (e.g., a current source) and a current shunt switch controlled by the window comparator. The array of modules is connected in parallel with the load. FIG. 1a shows a block diagram representative of a conventional digital switching regulator having a first (#1), a second (#2) and a third (#3) network connected in parallel to a bus line having a load coupled thereto. The networks #1, #2 and #3 have voltage regulation windows W1, W2, and W3 centered about setpoints Vset1, Vset2 and Vset3, respectively.
As shown in FIG. 1b, control of the bus (load) voltage is transferred between the first, second and third networks in response to variations in the bus current I.sub.bus. Hence, a disadvantage of conventional switching voltage regulators is that the nominal bus voltage about which regulation occurs varies in response to current drawn by the bus.
Moreover, this conventional digital approach is generally expensive to implement as the switches within the networks each require an associated window comparator. Additionally, the voltages with respect to which the window comparators are referenced are typically provided by a central amplifier. Hence, failure of this central amplifier results in a complete loss of regulation capability. This risk may be unacceptable in applications such as spacecraft bus voltage regulation which require a high degree of reliability.
An alternative digital switching regulator utilizing a shift register has recently been developed which overcomes certain limitations mentioned above in connection with conventional digital switching regulators. Specifically, Fleck et al. disclose a `voltate controlled solar array` in "10 kW Solar Array Switching Unit Performance Test Results", Proceedings of the 20th Intersociety Energy Conversion Engineering Conference, 1985. The regulator developed by Fleck includes an array of solar panels (current sources) connected to an array of shunt switches. The solar panels and shunt switches are connected in parallel with a bus and a load. When turned on, a given shunt switch diverts to ground the current from the solar panel connected thereto. Individual data registers within a shift register are operatively coupled to the shunt switches. A logic one or zero within each of the data registers turns the associated switch on or off, respectively. A comparator monitors the bus voltage and compares it with a desired reference voltage to generate a logic signal which is used to control the shift register and hence the shunt switches. Data within the shift register is updated in accordance with the logic signal in response to the application of a clock signal to the shift register. In this manner a relatively constant load current, and therefore bus voltage, is maintained by adjusting the current supplied to the bus and load in response to variations in the current drawn by the bus.
In the regulator developed by Fleck, the array of switches is controlled by a shift register and a single comparator. This implementation is generally more economical than that of the conventional digital switching regulators which have a comparator associated with each switch. However, in the regulator of Fleck both the clock signal and the logic signal generated by the comparator are continuously applied to the shift register. As a consequence, at least one bit within the shift register will change with each clock cycle even when the instantaneous bus voltage is substantially identical to the desired bus voltage. In this manner at least one switch changes state with every clock cycle even in the presence of a constant (steady state) bus current draw. This somewhat random switching results in a nonuniform ripple voltage and in typically increased power consumption.
In addition, in certain applications the regulators of Fleck may need to be combined in parallel to accommodate increased bus current variation. However, in such a parallel connection the nominal bus voltage about which regulation occurs becomes a function of the bus current in a manner substantially similar to that shown in FIG. 1b. That is, control of the bus voltage is transferred between the parallel connected regulators in response to changes in current drawn by the bus. Such an interdependence between the nominal bus voltage about which control occurs and the instantaneous bus current limits voltage regulation capability.
Hence, a need in the art exists for a digital switching voltage regulator for providing regulation about a single voltage despite variations in the current supplied thereby.